Do you mean that you are getting traffic from the internet (outside) to an SNMP Server on your inside network with a spoofed source address ?

Is the spoofed address a private address range that is getting through to your inside network ? If so you should really be doing some RFC1918 filtering on your firewall anyhow. Even though your ISP should be doing this at their edge device its good practice to put into the access list (at the top) a rule to block all private address ranges.

No. The SNMP server is in inside network. And we are monitoring the inside interface of PIX. The SNMP server is in UK and we have a Leased Line The SNMP server is having a public IP adress but it is not monitoring through Internet the traffic is coming through the Leased line.

I am getting a IP spoof between the IP address of PIX outside Interface and the SNMP server. Both of them are my IP address and it is a public IP address.